Brain imaging has helped researchers understand the role of dopamine in ADHD.Credit: JohnnyGreig/Getty
A couple of years ago, Jan Haavik was at a routine meeting of a research group that connects scientists studying attention deficit hyperactivity disorder (ADHD) with people who live with the condition. Haavik, a neuroscientist at the University of Bergen in Norway, recalls that one person with ADHD told him: “As everybody knows, we who live with ADHD have very low dopamine levels.”
Haavik was surprised to hear this because the scientific data do not suggest an unequivocal link between low levels of the neurotransmitter dopamine and ADHD. But the idea that low dopamine is a direct cause of ADHD is a common misconception, one that’s amplified on social media and even in popular books about the condition. The reality, Haavik and other researchers say, is that the causes of ADHD are more diverse and nuanced than a simple deficit in one chemical cue in the brain1.
Nature Outlook: ADHD
Links between dopamine levels and ADHD first emerged in the 1960s, when researchers found that certain stimulant drugs relieved symptoms of hyperactivity or inattention. The effectiveness of medications such as methylphenidate (Ritalin) and amphetamine (Adderall), which increase the levels of dopamine and other chemicals in the brain, led to the idea that ADHD was the result of a dopamine deficit, says paediatrics researcher James Swanson at the University of California, Irvine. But the definition was based only on the drugs’ mechanism of action, rather than any direct evidence of dopamine’s role in causing symptoms of ADHD, he says: “The drug corrects the symptoms, so the assumption is that patients have a deficit of neurotransmitters.”
The efficacy of stimulant treatments led researchers to use brain imaging, genetics and experimental models of ADHD to probe dopamine’s role in the condition. Over the years, this evidence coalesced into a framework that is commonly referred to as the ‘dopamine hypothesis’, which proposes that dopamine dysregulation is somehow associated with the condition. But precisely what the dysregulation is, and how it might cause the symptoms that people with ADHD experience, is still uncertain, says psychologist Hayley MacDonald of the University of Bergen in Norway.
The evidence shows that ADHD is a complex condition that cannot be defined simply as a lack of a single neurotransmitter. But researchers have demonstrated how aberrant dopamine signalling can lead to ADHD, and how other neurotransmitters such as serotonin might also be involved.
These advances could eventually lead to a better understanding of ADHD and pave the way to improved treatments. In the United States alone, around 6.5 million children and 15.5 million adults have a diagnosis of ADHD. Stimulant drugs are not equally effective for all those with the condition, however: some people don’t experience lasting benefits, and others struggle with side effects.
Recognizing that ADHD is “more than just a dopamine issue” is the first step towards better approaches to care, says Stephen Faraone, a psychologist and neuroscientist at Upstate Medical University in Syracuse, New York. If researchers can identify the biological mechanisms, he adds, “we may find a better place to intervene”.
Early evidence
... continue reading